Several methods for producing synthetic fuels have previously been developed. In particular, methods are known for the production of synthetic fuels from methanol.
In principle, these methods may be divided into two steps. In the first method step (olefin production), oxygenates are reacted to form preferably C2-C8 olefins, using a zeolite catalyst. In a second method step (oligomerization), the referenced olefins are converted to higher hydrocarbons at elevated pressure, using a zeolite catalyst. Between the two method steps, substreams composed of water and/or hydrocarbon may be separated out in a first separation step. After the second method step, in a second separation the higher hydrocarbons obtained are separated into a light fraction, a gasoline fraction, and a heavy fraction. After hydrogenation, the heavy fraction forms the diesel fuel end product, and with suitable separation, kerojet fuel as well. The liquefied petroleum gas (LPG) and heating gas products may be obtained from the light fraction.
The known methods considered below represent substeps of the above-described overall method.
The first method step (olefin production) is described, for example, in the production of in particular propylene from methanol in European patents EP 0 448 000 B1 and EP 0 882 692 B1 and in German patent applications DE 100 27 159 [U.S. Pat. No. 7,015,369] and DE 101 17 248 [Also U.S. Pat. No. 7,015,369]. The methanol to propylene (MTP) process, which employs a multistage fixed-bed reactor, is used in this method, and a pentasil-type zeolite is used as catalyst. To optimize product yield, a gas stream rich in unsaturated hydrocarbons is recirculated to the olefin reactor, and in this method step is separated from the reactor product. In addition, a portion of the water produced in the olefin production is recirculated. In the MTP process, however, the product sought in this method step is propylene, not olefins, for example.
The principle of the first method step (olefin production) is used in other patents for the production of gasoline. Cited here are U.S. Pat. Nos. 4,404,414, 4,788,369, and 4,035,430. The methods described therein are also carried out using multistage fixed-bed reactors loaded with zeolite catalyst (ZSM 4 or similar catalysts). Here as well, gas streams composed of saturated and unsaturated hydrocarbons (depending on the patent) are recirculated to the reactor. The purpose of recirculation is to adjust the temperature profile in the reactor and optimize the product yield of gasoline. However, unlike the previously referenced MTP process, in these patents no water is used as a recirculation stream.
The second method step (oligomerization), in which higher hydrocarbons are oligomerized from olefins and may then be hydrogenated to produce diesel fuel, is known from South African patents 9,101,969, 9,101,970, and 9,200,642. In this case, catalysts based on crystalline aluminum silicates of the pentasil type, in particular as described in European Patent 369 364 [U.S. Pat. No. 5,063,187], are used for the oligomerization. A multistage fixed-bed reactor is used here as well. This method is used primarily for producing diesel fuel from hydrocarbons which preferably contain C2 to C8 olefins. Also in this method, a recirculation stream is used for optimizing the product yield.
In addition, the combination of a method for producing olefins from methanol with a method for producing gasoline and diesel fuel from olefins is the subject matter of several patents.
In this regard, reference is made in particular to U.S. Pat. Nos. 4,579,999, 4,689,205, and 4,898,717. The cited patents describe a two-step process for producing diesel fuel and gasoline from methanol. The methanol to olefins (MTO) process is used in combination with the MOGD (Mobil olefins to gasoline/distillate) process for the oligomerization of olefins. All three patents use zeolite-type catalysts, and describe the preferred operating conditions for producing diesel fuel.
In the first two U.S. Pat. Nos. 4,689,205 and 4,898,717 [sic; U.S. Pat. No. 4,579,999], a fluidized bed reactor is used in the first method step. In addition, a substream from the gasoline product in the form of an unsaturated hydrocarbon stream is recycled to the first and second method steps. Ethylene and ethane are additionally recycled to the first method step.
In the third U.S. Pat. No. 4,898,717 a multistage fixed-bed reactor is used in both method steps. In this case, the first method step is carried out without recirculation. From the MTO reactor product of the first method step a C2 stream is separated as a by-product from which ethylene is obtained. This is meaningful in this process, since under the described operating conditions approximately 26% by weight of the MTO reactor product is composed of ethylene. This obviously reduces the overall conversion of methanol to diesel fuel.
The MTP process as well as the MTO process using a fluidized bed reactor are both characterized in that in the first method step the olefin production is carried out in the presence of a gas stream that is enriched with unsaturated hydrocarbons separated from the product stream of the first or second method step and returned to the first method step. The yield of propylene is thus optimized in the MTP process. In the MTO-MOGD process, unsaturated hydrocarbons are recirculated from the gasoline by-product, thereby increasing the product yield of diesel fuel and reducing the product yield of gasoline.
However, as a result of the recirculation of olefin-rich streams to the olefin reactor the olefins are partially converted into paraffins and other components. Paraffins and other components are no longer available in the oligomerization reactor for the formation of long-chain hydrocarbons, which in the end reduces the yield of diesel fuel.